Conductive materials such as carbon materials (e.g., carbon fibers, carbon nanotubes, graphenes, etc.) and metallic nanomaterials (e.g., metal nanoparticles, metal nanowires, etc.) are known. By using such a conductive material, developments of electric devices such as liquid crystal display and photoelectric conversion devices (e.g., organic EL devices, solar cells, optical sensors, etc.) have been made.
In the case of conductive materials obtained by using a carbon material, the used amount of rare metals or the like can be significantly reduced, or such a metal is not used at all in some cases. The conductive material using a carbon material also shows large mechanical strength in addition to high flexibility. Moreover, since this conductive material is chemically stable, it has drawn attention as a promising conductive material.
Although the conductive material obtained by using the carbon material has a relatively high electrical conductivity, the resistance in conduction between molecules is large. In the case where the conductive material is used as a transparent electrode with a large area, the electrical resistance becomes higher in the same light transmittance compared to an indium-tin oxide (ITO) film. When such a conductive material is used as a long distance wire or the like, the electrical resistance is further higher compared to a metal conductive material such as copper (Cu).
Since the metal nanomaterial has high conductivity, its composite in combination with a carbon material has been used for achieving an improvement in conductivity.
Electric devices such as liquid crystal display, solar cells, and organic EL devices have a pair of electrodes and a functional layer disposed between the pair of electrodes. At least one of the electrodes is a transparent electrode, and an ITO film is generally used as the transparent electrode.
As the negative electrode of the photoelectric conversion device such as a solar cell and an organic EL device, there have been used aluminum (Al) having a small work function and a magnesium (Mg) alloy having a much smaller work function. A photoelectric conversion device in which an ITO film is used for the negative electrode and a metal having a large work function is used for the positive electrode is also known, though its efficiency is low.
Indium (In), which is a rare metal, is used as the ITO film. As a stable and flexible transparent electrode that can be prepared at low cost without using the In, carbon nanotubes with an unsubstituted graphene structure and planar graphene thin films have been studied as the transparent electrode.